Method of treating refractory articles



Patented Nov. 21, 1950 UNITED STATES PATENT OFFICE METHOD OF TREATING REFRACTORY ARTICLES No Drawing. Application March 12, 1946, Serial No. 653,950

6 Claims. 1

This invention relates to a method of treating refractory articles, and more particularly to a method of treating refractory components of high frequency electric heating apparatus.

In the manufacture of certain electrical articles or devices, it is necessar to seal together hermetically in precisely predetermined spatial relation to each other a plurality of metal members and glass members, the principal joints being metal to glass. To effect this purpose blocks of refractory material are formed with accurately shaped and dimensional cavities so that when assembled together, the blocks present an internal chamber adapted to he in effect a jig to hold the metal and glass parts in accurately predetermined relation to each other. The blocks are also so formed that when so assembled they present an exterior form, e. g., cylindrical, adapted to be received into the interior of an electrical coil through which a high frequency electric current can be passed to heat both the refractory blocks and the metal members contained in the chamber. The refractory blocks forming the jigfurnace may preferably be made of lavite, a fine grained, relatively tough, natural lava mineral, although various suitably fired ceramic mate rials may also be used. A single apparatus as generally described above is satisfactory for laboratory or small volume and slow speed production. When, however, the attempt is made to compound several such apparatuses into an automatic, continuousl operating machine, difficulty arises due to the non-homogeneous character of the refractory material of the blocks. A pair or set of blocks made from one lump of commercial lavite, while generally and closely similar in character to another pair or set made from another lump, will, nevertheless, differ sufficiently in grain, chemical composition, or otherwise so that the same alternating field applied first to one, then to the other, for like periods of time, does not produce the same degree of heat in the two instances; and therefore the two sets of blocks cannot be used on the same automatic machine where successive cycles of operation must be alike in timing.

An object of the present invention is to provide a simple, reliable method of treating refrac ory articles such as the blocks above described to render the same alike in their electro-thermal conversion factor, i. e. their capacity per unit of volume to convert alternating electrical field energy into heat.

With the above and other objects in view, the invention may be illustratively embodied in the method of treating an article of iron bearing refractory material, which method comprises steps of firing the article at a temperature above that at which the article is to be subsequently employed, cooling the article, measuring the electro-thermic conversion factor of the article, and refi ing the article at a temperature predetermined in accordance with the measured value of the electro-thermic conversion factor thereof to increase the factor to a desired value.

Other objects and features of the invention will appear from the following detailed description of one embodiment thereof in a method of treating geometrically identically similar or symmetrical masses of lavite to render the same alike in efiiciency as walls of a furnace to be heated by a high frequenc electric field.

Commercial lavite is a fine grained, natural lava, a basic silicate of magnesium and aluminum with small amounts of iron also, and, when green or raw. contains some water, both as water of crystallization and as water entrapped among or adsorbed by the grains. When green or raW," i. e. as taken from the quarry or mine, it i machinable and can be formed with tools to fairly accurate size and shape. When so shaped, articles made from it can be fired, in any suitable kind of ceramic furnace, to drive off the water content, with due care to avoid too rapid heating lest evolved steam cause explosive fractures. Shrinkage in firing is small, usually about 1%. and predictable within small limits; and the fired material is durably hard and tough and capable of some degree of polish if desired and if not heated again beyond the temperature at which it was fired, takes on no further change of size or shape if used, as hereinbefore described, for wall segments or units of an electrically heated combination jig and furnace.

However, lavite is a natural product, and while different lots from the same source are generally similar in composition and microstructure, they are not identically alike. Blocks or segments made as described can be made substantially identically alik in form, density, hardness, etc, and yet, when put into service, are found to vary in electri al properties sufficientl to render it impossible to use them interchangeably or under conditions where uniformit of electrical char acteristics is essential. On microscopic examination, fired lavite shows a fairly regular distribution of tiny, usually globular, aggregates of iron oxide, apparently derived from the displacement of iron oxide from its basic silicate by magnesia or alumina, since free iron oxide does not appear to be present in green lavite.

' The electrical property of the material of particular interest here is the ability or capacity of the material per unit volum of its mass to convert the electrical energy of an alternating field into heat within the substance of the material. There appears to be no technical term in the art for this particular property. Hence, for convenience in the ensuing, it is proposed to use the phrase electro-thermal conversion factor to mean the ratio of electrical energy supplied into a volume unit of material submerged in an alternating electrical field to the heat energy appearing in the same volume unit per unit of time as a result of the interaction of the material and the field.

There already exists in those branches of electrical technology that deal with high frequency alternating fields, a, recognized quantity known as Q, frequently used as a measure of the dielectric value of a substance immersed in an alternating field, a definition and discussion of which can be found in any standard text on high frequency practice. The precise nature of Q is irrelevant here. The relevant fact is that the electro-thermal conversion factor above defined is inversely proportional to Q; and, since there exist Q meters, i. e. electrical instruments for measuring the Q of an article, also well known and fully described in the texts, the electro-thermal conversion factor of an article can be measured by using a Q-meter. One suitable instrument for this purpose is described in detail in U. S. Patent 2,137,787 of November 2-2, 1938, to H. A. Snow, to which reference may be had if desired.

Green lavite has too low and too unstable an electro-thermal conversion factor to be used in its natural state for elements of a jig-furnace. The changes effected in its substance by the temperatures involved in such use effect corresponding changes in thevalue of the factor. parts machined from green lavite, with due allowance for shrinkage in firing, are fired at a temperature sufficiently above operating temperatures subsequently to be employed, and for periods sufficiently long to render the electro-thermal conversion factor of the parts stable at the operating temperatures. For example, in the case of a jig-furnace for sealing glass to metal parts at temperatures of the order of 700 C., the lavite furnace elements can be rendered stable by firing them for an hour at 800 C.

. While any set of lavite parts shaped from one original mass of raw material and fired as described will have a stable electro-thermal conversion factor thereafter at temperatures u to its firing temperature, the value of the stabilized factor is not, in general, predeterminable to a close degree of approximation; and two sets identically alike in form and mass and shaped from different original lumps r even from different portions of one large lump of raw material, even if fired together, will not have identical values of the stabilized factor, and will not, in general, be interchangeable in use.

Applicant has discovered that the electro-thermal conversion factor stabilized in the manner described, can be controllably and predictably increased by a second firing at a temperature predeterminedly higher than the temperature of the original firing, provided this refiring be of sufficient duration. Thus, if two blocks of green lavite of identical shape and size be fired together at a given temperature and for a given time, they will, when tested with a Q meter, show different values for their electro-thermal conversion factors. If they are then refired together at Hence,

a higher temperature, test in the Q meter shows the factor of each to have been increased in the same ratio. They are still different from each other and from their former states, but the change in either is calculable from the change in the other, The electro-thermal conversion factor of a block of green lavite after an initial firing cannot be predicted beforehand; but the change in the factor effected by a refiring at a higher predetermined temperature can be pre dicted from an empiricall established table or graph of the results of similar firings on similar materials.

Thus, to make two or more sets of lavite articles, blocks, or elements which shall be of substantially identical and predetermined values of electro-thermal conversion factor, the shaped articles of green lavite are fired at a temperature and for a time known to experiment to stabilize the factor of the material at a value slightly less than the predetermined desired value. This firing may be done in an suitable furnace, and preferably in a reducing rather than an oxidizing atmosphere. The fired articles are then cooled to room temperature and their electro-thermal conversion factor measured by any suitable electrical measuring means appropriate to the purpose such as a Q meter, Each article is then individually fired again at that higher temperature, indicated b empirically established standards, which will increase the factor to the desired value.

To make the measurement desired, the instru ment shown in the above identified Snow patent may be connected to a coreless coil having a central space large enough to accommodate the article in question. The Q of the coil is measured once with its central space empt and again with the article therein, The difference of these measurements is a measure of the electro-thermal conversion factor of the article.

The essence of the invention consists in the discovery that, in the case of such material as commercial lavite, the electro-thermal conversion factor of a mass or article shaped from the material and fired to stabilize the factor for use at temperatures below the firing temperature, the factor can be predictably and controllably increased by refiring at a predeterminedly higher temperature. It also appears that material which is iron free, or substantially so, does not fall within the purview of the invention. Both the existence and the predictably modifiable character of the thermo-electric conversion factor appear to depend on the presence in the material of iron in some form which can be converted by firing from an electrically ineffective state to an electricall effective one. As noted above, green commercial lavite ordinarily contains a small proportion of iron, apparently present in the raw material as a silicate chiefly, and which, when the material is fired, appears as microscopically visible globules of iron oxid dispersed throughout the material, and apparently increasingly so as the temperature (and to a less extent the duration) of the firing is increased.

While the invention has been described s ecifically as applied to the case of lavite material because such material is easil and relatively chea oly obtainable as a natural product, the invention is not so limited. The method can be applied in like fashion to molded and fritted articles of steatite ceramic materials with a suitable admixture of iron or iron silicate material, or to arti--- cles of ball clay relatively high in iron with or without added magnesia and/or alumina to give the desired consistency, or to articles of other analogous natural or artificial materials,

What is claimed is:

l. The method of treating an article of iron bearing refractory material for use as a furnace element, which method comprises steps of firing the article at a temperature above that at which the article is to be subsequently employed but below a temperature approximately corresponding to a desired final electro-thermic conversion factor for the article, cooling the article, measuring the electro-thermic conversion factor of the article, and refiring the article at a still higher temperature predetermined in accordance with the measured value of the electro-thermic conversion factor thereof to increase the factor to the desired final value.

2. The method of treating a set of articles of iron bearing refractory material for use as furnace elements, which method comprises steps of firing the set of articles together at a temperature above that at which the articles are to be subsequently employed but below a temperature approximately corresponding to a desired final electro-thermic conversion factor for the articles, cooling the articles, measuring the electro-thermic conversion factor of the individual articles of the set separately, and refiring each article of the set individually at a still higher temperature predetermined in accordance with the measured value of the electro-thermic conversion factor thereof to increase the factor to the desired final value.

3. The method of treating an article of lavite for use as a refractory furnace element which method comprises steps of firing the article at a temperature above that at which the article is to be subsequently employed but below a temperature approximately corresponding to a desired final electro-thermic conversion factor for the article, cooling the article, measuring the electrothermic conversion factor of the article, and refiring the article at a still higher temperature predetermined in accordance with the measured value of the electro-thermic conversion factor thereof to increase the factor to the desired final value.

4. The method of treating an article of ceramic material containing iron for use as a refractory furnace element which method comprises steps of firing the article at a temperature above that at which the article is to be subsequently employed but below a temperature approximately corresponding to a desired final electro-therrnic conversion factor for the article, cooling the article, measuring the electro-thermic conversion factor of the article, and refiring the article at a still higher temperature predetermined in accordance with the measured value of the electrothermic conversion factor thereof to increase the factor to the desired final value.

5. The method of treating a set of articles of lavite for use as refractory furnace elements which method comprises steps of firing the set of articles together at a temperature above that at which the articles are to be subsequently employed but below a temperature approximately corresponding to a desired final electro-thermic conversion factor for the articles, cooling the articles, measuring the electro-thermic conversion factor of the individual articles of the set separately, and refiring each article of the set individually at a still higher temperature predetermined in accordance with the measured value of the electro-thermic conversion factor thereof to increase the factor to the desired final value.

6. The method of treating a set of articles of ceramic material containing iron for use as refractory furnace elements, which method comprises steps of firing the set of articles together at a temperature above that at which the articles are to be subsequently employed but below a temperature approximately corresponding to a desired final electro-thermic conversion factor for the articles, cooling the articles, measuring the electro-thermic conversion factor of the individual articles of the set separately, and refiring each article of the set individually at a still higher temperature predetermined in accordance with the measured value of the electro-thermic conversion factor thereof to increase the factor to the desired final value.

CHARLES E. BUCHWALD. JOHN EHRET, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,975,436 Sorrel Oct. 2, 1934 1,975,437 Sorrel Oct. 2, 1934 1,975,438 Sorrel Oct. 2, 1934 2,007,742 Brown July 9, 1935 2,137,787 Snow s Nov. 22, 1938 

1. THE METHOD OF TREATING AN ARTICLE OF IRON BEARING REFRACTORY MATERIAL FOR USE AS A FURNACE ELEMENT, WHICH METHOD COMPRISES STEPS OF FIRING THE ARTICLE AT A TEMPEATURE ABOVE THAT AT WHICH THE ARTICLE IS TO BE SUBSEQUENTLY EMPLOYED BUT BELOW A TEMPERATURE APPROXIMATELY CORRESPONDING TO A DESIRED FINAL ELECTRO-THERMIC CONVERSION FACTOR FOR THE ARTICLE, COOLING THE ARTICLE, MEASURING THE ELECTRO-THERMIC CONVERSION FACTOR OF THE ARTICLE, AND REFIRING THE ARTICLE AT A STILL HIGHER TEMPERATURE PREDETERMINED IN ACCORDANCE WITH THE MEASURED VALUE OF THE ELECTRO-THERMIC CONVERSION FACTOR THEREOF TO INCREASE THE FACTOR TO THE DESIRED FINAL VALUE. 